This invention relates to a shutter control device for a camera and a control device therefor, and particularly to a device and a method for controlling the exposure amount and opening time of a shutter for a camera in a diaphragm-priority mode.
An exposure amount (Ev) of a camera is generally dependent on the diaphragm value of the shutter and the opening time therefor, that is, the exposure amount is a function having the diaphragm value of the shutter and the opening time therefor as independent variables. Therefore, the opening time of the shutter is determined by setting the desired exposure amount and the diaphragm value.
An operation of driving a shutter is schematically shown in FIG. 3 wherein the abscissa and ordinate represent the time and aperture area, respectively. In FIG. 3, the shutter starts its opening motion at a point A, is fully opened at a point B, starts its closing motion at a point C after an opening time t elapses, and then is completely closed at a point D. The area of the trapezoid which is enclosed and surrounded by the points A, B, C and D, corresponds to the exposure amount (Ev) for this shutter driving operation. The shutter driving operation which provides such a trapezoidal waveform for the opening and closing operations of the shutter is referred to as "a trapezoid control".
In a conventional method for controlling the opening time of the shutter in which the exposure amount is controlled by a stepping motor, etc., the relationship between the exposure amount (Ev) and the opening time is stored beforehand as a data table as shown in FIG. 6 in a storing unit of a control device, and the opening time for a desired shutter driving operation is determined on the basis of the data table. For example, a data table as shown at the left side of FIG. 6 is used for a shutter driving operation for a full-opened diaphragm (hereinafter referred to as "diaphragm full-opening operation").
When the trapezoid control is carried out in the diaphragm-priority mode, the opening time is varied in accordance with the diaphragm value. For example, if the diaphragm value is set at a value smaller than the value for the full-opened diaphragm in the graph of FIG. 3, the opening time must be set to be longer to obtain the same exposure amount as set for the diaphragm full-opening operation. Therefore, in order to control the opening time with the data table, a data table must be individually prepared in accordance with each of various diaphragm values, for example, diaphragm values such as F4, F5.6, F8, etc., and thus the number of the data tables is greatly increased. In addition, the linearity of a trapezoidal waveform representing opening and closing operations of a shutter for a camera is ordinarily dispersed in accordance with each shutter, and it has been hitherto very difficult to correct the dispersion of the linearity of the trapezoidal waveform.
As previously set forth, the exposure amount (Ev) of a camera is generally determined by the area of a waveform (trapezoidal waveform). FIG. 8 represents a series of shutter driving operations for a period when a shutter starts its opening motion, reaches a predetermined aperture and completed its closing motion. In the so-called trapezoid (trapezoidal waveform) control operation in which the exposure time is controlled on the basis of a predetermined diaphragm value, the shutter is controlled to be opened at a predetermined aperture for a predetermined opening period, and thus an AE pulse data and an analog time data used to determine a time between pulses must be obtained. Therefore, a conventional camera has been equipped with a storing unit in which a data table representing the number of AE pulses and another data table representing an analog opening time are stored for each and every exposure amount (Ev) in accordance with an aperture, and the control of the exposure amount has been carried out with reference to these data tables.
However, such a conventional AE control system requires a data table to be prepared for each and every exposure amount in order to cover all aperture values, and thus the number of the data tables to be prepared is greatly increased. In addition, the linearity of the trapezoidal waveform representing opening and closing states of the shutter is ordinarily dispersed in accordance with a shutter of a camera, and it has been conventionally very difficult to correct the dispersion of the linearity.